1. Field of the Invention
This invention relates to an exposure control device for a color printer, and more particularly to an exposure control device for stepwisely controlling the amount of light used for exposing photographic paper in a color photographic printer.
2. Description of the Prior Art
In a color photographic printer, the intensity of light source cannot be changed because the color balance of the light source spectrum must be maintained constant. Therefore, the intensity of the light source is made constant and the exposure time for the three primary colors, cyan (C), magenta (M) and yellow (Y), is controlled by use of filter shutters of the three colors.
In the color printer, the exposure time is set to be 200 msec for standard color negatives, 800 msec for two step over-exposure negatives, and 70 msec for two or three step under-exposure negatives, for instance. Recently, the speed of the printer is increased and it is desired that the exposure time be shortened to a half of the conventional time exposure. In such a case, the exposure time is required to be as short as 30 msec for the under-exposure negatives. The 30 msec is too short for the electrical and mechanical shutter control means to conduct the normal operation of the filter shutters in the conventional printers. This results in malfunction of the exposure control means, and the exposure time cannot correctly be controlled.
In order to avoid the above mentioned difficulty, it is known to elongate the exposure time only when the prints are made from under-exposure negatives. This is carried out by inserting a neutral density filter into the optical path of the printer when an under-exposure negative is brought to the printing station of the printer. The insertion of the neutral density filter is simply controlled by turning ON and OFF a control circuit. Therefore, the amount of light used for exposing the photographic paper is abruptly changed at a predetermined level of density of the negatives. Accordingly, the exposure control or density compensation cannot be conducted continuously, which results in production of color prints of two groups of color balance which are much different from each other. In order to reduce the difference in color balance of these color prints, it is necessary to use a complicated color balance correcting circuit which is practically almost impossible.